The present invention relates to imaging systems and, more particularly, to optical-coherence imaging systems.
A variety of approaches to imaging using optical coherence tomography (OCT) are known. Such systems may be characterized as Fourier domain OCT (FD-OCT) and time domain OCT (TD-OCT). FD-OCT generally includes swept source (SS) and spectral domain (SD), where SD systems generally use a broadband source in conjunction with a spectrometer rather than a swept laser source and a photodiode(s). TD systems generally rely on movement of a mirror or reference source over time to control imaging depth by providing coherence depth gating for the photons returning from the sample being imaged. Both systems use broadband optical sources, producing a low aggregate coherence that dictates the achievable resolution in the depth, or axial, direction.
These imaging techniques are derived from the general field of Optical Low Coherence Reflectometry (OLCR). The time domain techniques are specifically derived from Optical Coherence Domain Reflectometry. Swept source techniques are specifically derived from Optical Frequency Domain Reflectometry. Spectral domain techniques have been referred to as “spectral radar.”
In contrast to time domain systems, in FD-OCT the imaging depth may be determined by Fourier transform relationships between the acquired spectrum, rather than by the range of a physically scanned mirror, thereby allowing concurrent acquisition of photons from all imaged depths in the sample. Specifically, in FD-OCT, the optical frequency interval between sampled elements of the spectrum may be used to control the imaging depth, with a narrower sampling interval providing a deeper imaging capability.
In addition to total bandwidth, which generally controls the axial resolution, and sampling interval, which generally controls the imaging depth, a third parameter, the effective sampled linewidth, generally controls a quality of the image as function of depth. As used herein, references to “linewidth” refer to the effective sampled linewidth unless indicated otherwise. As the effective sampled linewidth at each sampled interval is increased, the effective sampled coherence length decreases, which may produce a detrimental envelope of decreasing signal-to-noise ratio across the imaged depth. This behavior is commonly known as fall-off and it is generally desirable to minimize this signal quality fall-off.